Throughout the ages, we have sought to improve the level of comfort offered by our surroundings. In colder regions, we have tried to heat our dwellings and in warmer climates, to cool them down because if we are not comfortable, we can neither work nor relax.

Heat always flows from a warmer substance to a cooler one. In reality, the faster moving molecules transfer some of their energy to slower ones. Thus the faster molecules slow down a little and the slower ones speed up. Expressed simply, this means that when it is hot outside, the heat attemps to 'invade' colder interior spaces.